Organic electroluminescent (hereinafter, the term of electroluminescent may be abbreviated as EL) elements that interpose an organic layer between a pair of electrodes and applies a voltage between both electrodes to emit light have advantages such as high levels of visibility due to self-luminescent colors, excellent impact resistance because of being all-solid elements unlike liquid crystal elements, high response speeds, to be less affected by change in temperature, and wide viewing angles, and the use thereof as light emitting elements in display devices and lighting devices has been attracting attention.
As the organic EL elements, it is known that multiple layers of organic layers such as a hole injection and transport layer, a light emitting layer, and an electron injection and transport layer are stacked between an anode and a cathode. Vapor deposition methods and application methods have been widely adopted commonly as methods for forming the organic layer films.
The vapor deposition methods have the advantage of easily stacking layers and thus being able to construct a functionally separated multi-layer structure, thereby achieving increased efficiencies and lifetimes. However, it is difficult to uniformly control film thicknesses in large areas, and the formation of large-area organic layers has the problems of requiring long periods of time, thus resulting in a poor production efficiency, and requiring a large-scale vacuum system, thus resulting in high manufacturing cost. In addition, for example, in the case of forming an organic layer film by co-deposition of a host of an organic compound and a dopant of a metal or a metal compound, there is a possibility that the lifetime will be shortened due to the organic compound decomposed during the deposition, or decomposed and mixed in the film, because the deposition temperature of the metal or metal compound is higher than the deposition temperature of the organic compound.
It is to be noted that, Patent Literature 1 discloses the use of a metal borate or a metal organic boron compound as an electron injecting material that is comparable in dopant deposition temperature to organic compounds and relatively unlikely to damage organic compounds.
On the other hand, the application methods are advantageous in terms of cost, and also have the advantage of easily achieving larger areas, as compared with the vapor deposition methods. In addition, organic compounds are not decomposed during film formation as observed in the vapor deposition methods. Therefore, various methods for manufacturing organic EL elements have been proposed in which organic layers are formed by application methods. Above all, electron injection and transport layers formed on light emitting layers are formed mainly by vapor deposition methods, and studies have been thus made for forming electron injection and transport layers by application methods (see Patent Literatures 2 and 3). It is to be noted that in the application methods, materials have to be dissolved or dispersed in solvents in order to use coating liquids, materials for use in vapor deposition methods are not able to be directly diverted, and it is thus necessary to select materials appropriately.
For example, Patent Literature 2 proposes a method in which an electron injecting layer is formed by a wet process with the use of a non-ionic organic metal complex or a metal phthalocyanine. Patent Literature 3 proposes the formation of an electron injecting organic layer by application with the use of an organometallic salt or an organometallic complex compound. Moreover, although not for electron injection and transport layers, Patent Literature 4 proposes a method of forming a light emitting layer by a wet process with the use of a non-ionic organometallic complex as an electron transporting luminescent agent.
Then, in recent years, organic boron compounds have been attracting attention as materials for organic EL elements, because of being relatively stable against water and air.
For example, Patent Literature 5 proposes the use of an organic boron compound as a luminescent substance for emitting blue light of high color purity.